Whole genome sequence analysis of the 2018 Persian onager isolate suggests sublineages within the Taylorella asinigenitalis species.

Objective Overview

• This research article analyzed the whole genome sequence of a Taylorella asinigenitalis strain isolated from a Persian onager in the UK, revealing genetic sublineages within the T. asinigenitalis species.
• The study combined genomic and phenotypic data to assess the strain’s characteristics and its relationship to other known T. asinigenitalis strains, suggesting greater genetic diversity than previously recognized.

Introduction and Context

• In 2018, a Taylorella asinigenitalis strain named MCE663 was isolated from a Persian onager in a UK zoo. This marked the first such identification in the UK.
• Contagious equine metritis (CEM) is typically caused by Taylorella equigenitalis, but a 2019 report identified T. asinigenitalis ST70 as capable of causing endometritis naturally, indicating pathogenic potential previously unconfirmed for this species.
• Multilocus Sequence Typing (MLST) of the MCE663 strain classified it as a new, atypical sequence type (ST60), indicating genetic divergence from other known strains.

Research Objectives

• The study aimed to sequence the entire genome of the MCE663 strain to characterize its genetic makeup more precisely.
• Phenotypic characterization, including colony morphology and antibiotic susceptibility, was also performed to complement genetic analysis.
• The goal was to understand the strain’s placement within the T. asinigenitalis species and investigate possible sublineages or subspecies.

Methodology

• Whole genome sequencing was conducted on the MCE663 strain to generate its complete genetic profile.
• Phenotypic tests included colony size and antibiotic susceptibility assays to determine physical and functional traits.
• Comparative genome analyses were done against three published T. asinigenitalis genomes to assess average nucleotide identity (ANI) and phylogenetic relationships.

Key Results

• MCE663 exhibited typical identification features of T. asinigenitalis but formed smaller colonies compared to other strains.
• The strain was susceptible to all antibiotics tested, indicating no detected resistance.
• Phylogenetic analysis at the genome level showed that MCE663 belonged to a distinct phylogroup separate from the other known T. asinigenitalis genomes.
• The ANI between MCE663 and the three other published genomes was approximately 96.1%, while ANI among those three was around 98.3%.
• According to the current species delineation cut-off of 95% ANI and subspecies cut-off of 98% ANI, MCE663’s genome identity suggests it represents a sublineage within the T. asinigenitalis species rather than a new species.

Conclusions and Implications

• This study provides the first evidence of distinct genetic sublineages within the T. asinigenitalis species, supporting a revision of its genomic diversity and taxonomy.
• The findings raise awareness about the potential pathogenicity of different T. asinigenitalis sublineages, especially in light of the recent outbreak linked to ST70.
• Detection of a new atypical strain in the UK also underscores the importance of ongoing surveillance of Taylorella species in equine populations and wildlife reservoirs.
• Future research could explore the clinical significance, host specificity, and evolutionary history of these sublineages to improve understanding and control of contagious equine metritis and related infections.